1. Field of the Invention
The present invention is directed to a method for acquiring time-resolved and location-resolved three-dimensional data sets by magnetic resonance, as well as to an apparatus for implementing the method.
2. Description of the Prior Art
The representation of blood vessels (angiography) is currently being increasingly implemented with MR technology. An MR contrast agent (for example, GdDTPA) is often utilized for this purpose in conjunction with a shortening of the T1 time, so that the spread of the contrast agent can be tracked well with the assistance of highly T1-weighted pulse sequences. Typically, the contrast agent is intravenously injected. The examination is implemented as soon as the contrast agent is located in specific arteries. Given the standard contrast agent dose and an injection rate of 2 ml/s, the injection is carried out over a time span of, for example, 8-12 seconds. The MR measurement must ensue when the contrast agent is flowing through the vessel section to be examined. When, for example, the contrast agent has already spread in veins or in tissue sections that lie in the observation window, a diagnosis on the basis of the angiography examination becomes considerably more difficult. An exact timing of the MR measurement is not entirely simple, particularly because the time between the beginning of the contrast agent injection and the appearance of the contrast agent in the vessel section to be examined (also referred to as transit time) varies dependent on the vascular section to be examined and also differs from patient to patient. Therefore, a fixed time difference between contrast agent injection and measurement can therefore be employed; rather, the point in time of the measurement must be individually matched to the examination. These relationships are explained, for example, in U.S. Pat. Nos. 5,417,213 and 5,553,619 and 5,579,767 and 5,590,654.
Heretofore, one of the following methods was usually employed for synchronization between contrast agent injection and measurement:
A prior information about the average transit time: The transit time to a specific vascular section can be roughly empirically estimated. Additionally, age or other physiological information about the patient can also be considered in order to correctly estimate the transit time. Dependent on the experience of the examining person and on the individual measuring situation, unusable measurements or even misinterpretations, however, can occur.
Measuring the transit time with the assistance of a test bolus: A small test bolus (for example, approximately 1 cm3) of the contrast agent is employed in order to determine the actual transit time. This in fact leads to more precise measurements but lengthens the measuring time and increases the outlay for the operator of the system.
It is also known to use a fast, two-dimensional MR sequence to observe when the contrast agent arrives in an area in the observation window or just before the observation window. As soon as the examining person determines the arrival of the contrast agent, the start signal is given for the actual angiography measurement in the form of a 3D measuring sequence.
The aforementioned measuring sequence can also be automated in that the MR system recognizes a signal change due to the entry of the contrast agent in a test measuring window and then automatically triggers the angiography measurement with a 3D measuring mode.
In addition to exact timing, another problem in the above technique is that only one image is obtained in a predetermined measuring window. If a vessel, particularly in the case of a vascular illness, does not fill with contrast agent until later, this can lead to a misdiagnosis.
It is also possible to implement a series of 3D measurements following one another fast enough, with especially fast gradient echo sequences, so that not only the condition of the spread of the contrast agent at a predetermined point in time that is identified, but also the flow behavior of the contrast agent bolus into an observation window or, in more general terms, the contrast agent dynamics in the vessels under observation that can be acquired. The diagnostic value is thus considerably expanded, and the aforementioned problems with the exact timing of the measurement are eliminated. For example, a repetition time of 3.2 ms is currently achieved with fast gradient echo sequences having extremely short echo times. The measuring time Tacq for the measurement of a complete 3D dataset is derived according to the following equation:
Tacq=TRxc2x7Npxc2x7Ns
wherein Tacq is the measuring time, TR is the repetition time, Np is the number of phase-coding steps in a phase coding direction, and Ns is the number of phase-coding steps in slice selection direction. When, for example, Np=100 and Ns=24 are selected, then the measuring time for a 3D dataset is 7.75, with the aforementioned repetition time of TR=3.2 ms. An adequate separation of arterial and venous phases is generally achieved by means of a series of such measurements, for example in the carotid artery. The spatial resolution is established by the number of phase coding steps in phase coding direction or slice selection direction. It follows from the above equation that the measuring time becomes longer, i.e. the time resolution becomes poorer, the better the spatial resolution is. Such measurements are therefore always a compromise between time resolution and spatial resolution.
German PS 43 27 325, corresponding to U.S. Pat. No. 5,474,067, discloses a technique for shortening the measuring time for motion events without having to accept losses in the time resolution. The k-space can thereby be divided into individual segments. With the acquisition of a number of filed raw datasets at different times of a motion sequence, signals of the middle segment are employed in common for two chronologically successive raw datasets, i.e. the middle segment is measured more often than the other segments. The fact is exploited that the image contrast of the image reconstructed from the raw datasets is predominantly determined by the center of the k-space.
Although the time resolution and/or spatial resolution of MR measurement is already clearly improved with this method, the spatial or time resolution is still inadequate in many applications, particularly given 3D datasets.
It is known, for example from the article, xe2x80x9cAre the corners of k-space worth preserving?"", by M. A. Bernstein, Proceedings of the Society of Magnetic Resonance, 1995, Vol. 2, page 734, that the xe2x80x9ccornersxe2x80x9d of the k-space, i.e. data that lie outside a circle, can be discarded in the image reconstruction without greater sacrifices in quality.
According to an article, xe2x80x9cA circular echo planar pulse sequencexe2x80x9d, by J. M. Pauly et al., Proceedings of the Society of Magnetic Resonance, 1995, page 106, this fact is used for scanning a circular area of the k-space with an EPI sequence. The lines of the raw data matrix are thereby shortened toward the edges of the k-space in readout direction.
German OS 43 17 028, corresponding to U.S. Pat. No. 5,754,046, discloses a modified keyhole scanning method of a three-dimensional k-space. The k-space is subdivided into cuboid sub-areas, each cuboid covering the entire k-space in the z-coordinate direction. Sub-areas of the overall dataset that change from measurement to measurement are updated in a predetermined sequence.
German OS 197 13 005 discloses implementing the phase coding of the nuclear magnetic resonance signals, for contrast agent tracking with MR imaging, such that measured values in the central area of the k-space are acquired at earlier times than measured values in edge regions of the k-space.
Segmenting does not take place in this method.
An object of the present invention is to implement a method or an apparatus for acquiring a number of three-dimensional MR datasets such that the spatial resolution and/or time resolution of the measurement is improved.
This object is achieved in a method and apparatus according to the invention, wherein nuclear spins are excited in an examination subject and nuclear spins are phase coded in first and second phase coding directions by respective phase coding gradients, and wherein the nuclear spins are read out from the subject under a readout gradient in a further direction, and wherein the phase coding and readout steps are repeated for respectively different combinations of phase coding in the phase coding directions, so that a number of three-dimensional data sets, occupying three-dimensional k space in respective segments, are obtained, these segments including a central, cylindrical segment and a number of annular cylindrical segments successively surrounding the central cylindrical segment, each of the segments having a longitudinal axis in the readout direction, and wherein the three-dimensional datasets are respectively allocated to a number of different time spans, with the different combinations of the phase coding gradients having a chronological sequence so that data occupying the central k space segment are acquired more often than data occupying the annular segments.
In accordance with the inventive method and apparatus, therefore, a reduction of the amounts of data measured per 3D dataset occurs in two respects, so that a higher time/spatial resolution than in conventional methods and devices is obtained. First, the lines in the z-direction within in the xe2x80x9ccornersxe2x80x9d of the 3D k-space are not measured, which has practically no negative effect on the image quality for reasons that are known. Second, due to the segmenting, only a cylindrical region of the k-space is measured anew for each 3D dataset, whereas the other, annular segments are employed in common for successive 3D datasets. Compared to the segmenting into cuboid areas of the 3D k-space disclosed by the aforementioned German OS 43 27 325, the annular segmentation in accordance with the invention is adapted better and thus leads to an improved spatial/time resolution.
In an embodiment, an interpolation of the measured values preceding and following the measuring time window of the central segment in terms of time is implemented in segments outside the central segment. The spatial/time resolution thus can be improved further.
A further reduction in the amounts of measured data without significant losses in spatial resolution is obtained in an embodiment wherein substantially only half, or even only one-fourth, of the 3D k-space that is measured. The fact is exploited that the raw data matrices are conjugate-symmetrical relative to the axes, so that one quadrant or one half of the overall raw dataset contains the complete information. Employment of the method in MR angiography is especially advantageous since it allows the spread of the contrast agent in the blood stream to be observed very well.